1. Field of the Invention
This invention relates to a printer with a lens in its exposure system, and more particularly to a conjugate length varying device for altering the print size.
1. Background Art
A printer often uses a lens in its exposure system to transfer an image framed on a negative film or on a CRT (cathode ray tube) onto a color print paper. In this case, if it is required to change the print size on the print paper, two methods are generally employed.
In one of the methods, a plurality of lenses having different focal lengths from one another are provided in the exposure system. The lenses are selectively used according to the size of an image be formed on the color paper. For instance, these lenses are arranged on a turret type lens mount, and when it is required to change the print size, the lens mount is turned so that one of the plurality of lenses is selected for the new print size and this lens is set in the optical path.
In the other method, a single lens is employed in the exposure system, and the lens conjugate length is controlled to change the print size. That is, in order to change the print size, the printer easel and the lens are moved along the direction of the optical axis so as to change the conjugate length between the negative film or the CRT and the projection surface of the color paper.
However, the first method is disadvantageous in the following points. Since it is necessary to provide a plurality of lenses in the exposing system, an apparatus for practicing the method is unavoidably high in manufacturing cost. Furthermore, since the apparatus needs the turret type lens mount for selectively using the lenses, the apparatus is necessarily intricate in construction, which also increases the manufacturing cost of the apparatus.
The second method also suffers from difficulties. Namely, it is necessary to provide means for moving the printer easel up and down so that an apparatus for practicing the method is also intricate in construction and high in manufacturing cost.